This disclosure relates to an array of airfoils within a stage of a gas turbine engine, for example, a turbine stator vane array.
A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustor section where it is mixed with fuel and ignited to generate a high-speed exhaust gas flow. The high-speed exhaust gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section typically includes low and high pressure compressors, and the turbine section includes low and high pressure turbines.
The compressor and turbine section includes circumferential arrangements of fixed and rotating stages. Structural vibratory coupling between circumferentially and axially adjacent airfoils can occur during engine operation. For rotating stages of the engine, blade mistuning has been used in which there are two sets of blades arranged in circumferentially alternating relationship to provide an even numbered blade array. One set of blades has a different characteristic than the other set of blades to provide two different resonant frequencies. For fixed stages, vanes have been mistuned by providing different sets of vanes in adjacent quadrants of the array.
A significant driver for turbine blade vibration is the pressure distortion produced by nearby turbine vanes. Each successive vane passing by produces a pressure fluctuation on the blade such that if the product of the number of pressure disturbances per revolution and the rotational speed of the blade lines up with a fundamental frequency of the blade, then a vibratory response leading to potential high-cycle fatigue (HCF) failure could result. One technique for mitigating this effect is to use two sectors of vanes each of which contain differing vane geometries and number of vanes from each other, though the vane spacing within each sector is uniform and equal to 360 degrees divided by an integer representing the vane count if the pattern were extended around the entire annulus. This has the effect of reducing the strength of the periodic pressure disturbance from the vanes at the original frequency of concern.